Nitric Oxide Inhibits ATP-Dependent CA2+Uptake into Platelet Membrane Vesicles

The reduction by nitric oxide donors of Ca2+mobilization in stimulated platelets lead us to investigate the direct effect of authentic NO on ATP-dependent Ca2+uptake into platelet membrane vesicles. The effects of NO were compared to those of thapsigargin and 2,5-di-(t-butyl)-1,4-benzohydroquinone, two specific inhibitors of the sarco/endoplasmic reticulum Ca2+-ATPases. All three compounds modulated the initial rate of ATP-dependent Ca2+uptake. NO effects on the initial rate of active Ca2+uptake were biphasic, with an inhibition above 10 nmol/L and a stimulation below this concentration. These effects could not be attributed to cGMP, its usual effector molecule, or to nitrite ions, its metabolic product. NO inhibitory effects were decreased after a five min incubation, indicating that they were due to a short-lived compound and reversible. These results suggest that NO is functionally coupled to SERCA pumps of the dense tubular system through a cGMP-independent mechanism.